<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

  <p>Drag the bar to change its width:</p>
  <div style="background: orange; width: 60px; height: 20px">
  </div>
  <script>
    let lastX; // Tracks the last observed mouse X position
    let bar = document.querySelector("div");
    bar.addEventListener("mousedown", event => {
      if (event.button == 0) {
        lastX = event.clientX;
        window.addEventListener("mousemove", moved);
        event.preventDefault(); // Prevent selection
      }
    });
  
  
    function moved(event) {
      if (event.buttons == 0) {
        window.removeEventListener("mousemove", moved);
      } else {
        let dist = event.clientX - lastX;
        let newWidth = Math.max(10, bar.offsetWidth + dist);
        bar.style.width = newWidth + "px";
        lastX = event.clientX;
      }
    }
  </script>
      
</body>
</html>